Method of injection molded flip chip encapsulation

ABSTRACT

The electrical interconnections between an integrated circuit chip assembly are encapsulated and reinforced with a high viscosity encapsulant material in a single step molding process wherein a mold is placed over an integrated circuit chip assembly and encapsulant material is dispensed through an opening in the mold and forced around and under the integrated circuit chip by external pressure encapsulating the integrated circuit chip assembly. An integrated circuit chip assembly having a reinforced electrical connection which is more resistant to weakening as a result is stress created by differences in coefficient of thermal expansion between the integrated circuit chip and the substrate to which the integrated circuit chip is attached is produced.

BACKGROUND OF THE INVENTION

[0001] This invention relates to an improved method for encapsulatingand reinforcing the electrical interconnections between an integratedcircuit chip and a substrate. It also relates to an integrated circuitchip assembly produced by said method.

[0002] An integrated circuit chip assembly generally comprises anintegrated circuit chip attached to a substrate, typically a chipcarrier or a circuit board. The most commonly used integrated circuitchip is composed primarily of silicon having a coefficient of thermalexpansion of about 2 to 4 ppm/° C. The chip carrier or circuit board istypically composed of either a ceramic material having a coefficient ofthermal expansion of about 6 ppm/° C., or an organic material, possiblyreinforced with organic or inorganic particles or fibers, and having acoefficient of thermal expansion in the range of about 6 to 50 ppm/° C.One technique well known in the art for attaching the chip to thesubstrate is flip chip bonding. In flip chip bonding, a pattern ofsolder balls, usually having a diameter of about 0.002 to 0.006 inches,is formed on one surface of the integrated circuit chip, fully orpartially populating the active chip surface with interconnection sites.A matching footprint of solder wettable terminals is provided on thesubstrate. The integrated circuit chip is then placed in alignment withthe substrate, and the chip to substrate connections are formed byreflowing the solder balls. During operation of an integrated circuitchip assembly, cyclic temperature excursions cause the substrate and theintegrated circuit chip to expand and contract. Since the substrate andthe integrated circuit chip have different coefficients of thermalexpansion, they expand and contract at different rates causing thesolder ball connections to weaken or even crack as a result of fatigue.To remedy this situation, it is common industry practice to reinforcethe solder ball connections with a thermally curable polymer materialknown in the art as an underfill encapsulant.

[0003] Underfill encapsulants are typically filled with ceramicparticles to control their rheology in the uncured state, and to improvetheir thermal and mechanical properties in the cured state. Underfillencapsulants have been used widely to improve the fatigue life ofintegrated circuit chip assemblies consisting of an integrated circuitchip of the flip chip variety attached to a substrate comprised of analumina ceramic material having a coefficient of thermal expansion ofabout 6 ppm/° C. More recently, integrated circuit chip assemblies havebeen manufactured using substrates comprised of a reinforced organicmaterial having a composite coefficient of thermal expansion of about 20ppm/° C.

[0004] The underfill encapsulation process is typically accomplished bydispensing a liquid encapsulant directly onto the substrate at one ormore points along the periphery of the integrated circuit chip. Theencapsulant is drawn into the space between the integrated circuit chipand the substrate by capillary forces, and forms a fillet around theperimeter of the integrated circuit chip. The diameter of the fillerparticles in the encapsulant is typically smaller than the height of thespace so that flow is not restricted, with typical encapsulants havingviscosities of about 10 Pa-s at the dispense temperature. Once theunderfilling process is completed, the encapsulant is heat cured in anoven. Cured encapsulants typically have coefficients of thermalexpansion in the range of 20 to 40 ppm/° C. and a Young's Modulus ofabout 1 to 3 Gpa, depending on the filler content and the type ofchemistry. Depending on the materials the integrated circuit chip andthe substrate are composed of, it may be desirable to further alter thecured properties of the encapsulant. However, the requirement that theencapsulant have low viscosity in the uncured state so that it flowsreadily into the space between the integrated circuit chip and thesubstrate severely restricts the formulation options. For example, theaddition of more ceramic filler would result in a lower coefficient ofthermal expansion, but would cause an increase in the viscosity of theuncured encapsulant. Furthermore, even with the use of underfillencapsulation, fatigue life of an integrated circuit chip assembly isshorter when the integrated circuit chip is interconnected to an organicsubstrate as opposed to a ceramic substrate due to the greater mismatchin thermal expansion between the typical integrated circuit chip andorganic substrates.

[0005] Also known in the art is a method wherein a package body isformed around the perimeter of the flip chip using a two step process.First the integrated circuit chip assembly is underfilled as describedabove. Next, a package body is formed around the perimeter of theintegrated circuit chip using a molding process.

[0006] The prior art also suggests a process wherein additionalreinforcement is achieved by forming a package body around theintegrated circuit chip assembly using a single step operation. In thisprocess, a large opening of about 50% of the size of the integratedcircuit chip is formed in the substrate under the integrated circuitchip. This approach essentially eliminates the space between theintegrated circuit chip and the substrate that is typical of aconventional integrated circuit chip to substrate interconnection, buthas the drawback of limiting the active surface area of the integratedcircuit chip that can be utilized for forming interconnections becauseonly the perimeter of the active surface of the integrated circuit chipcan be used.

[0007] It is an object of the present invention to provide a method ofencapsulating and reinforcing the electrical interconnections of anintegrated circuit chip assembly which allows the use of highly viscousencapsulating materials and eliminates the need to use differentencapsulating materials for underfilling and overmolding. It is also anobject of this invention to provide a method of encapsulating anintegrated circuit chip assembly which enables simultaneous underfillingand overmolding without reducing the active interconnection area of theintegrated circuit chip or substantially altering the substrate design.Another object of this invention is to provide an integrated circuitchip assembly having a reinforced electrical interconnection which ismore resistant to weakening as a result of stress created by thedifferences in coefficient of thermal expansion between the integratedcircuit chip and the substrate.

SUMMARY OF THE INVENTION

[0008] This invention provides an improved method for encapsulating thesolder ball interconnections of an integrated circuit assembly whichaccommodates the use of high viscosity encapsulating materials andallows simultaneous underfilling and overmolding; and eliminates theneed for a dam to contain flow. In accordance with the preferredembodiment of this invention, an integrated circuit chip assemblycomprised of an integrated circuit chip mounted on a chip carrier ordirectly on a circuit board in a standoff relationship by solder ballconnections is provided.

[0009] A mold is placed over the integrated circuit chip. The mold isconstructed with an opening extending from the inside surface of themold to the outside surface of the mold and at least one vent. Externalpressure is applied to the mold to seal the mold to the surface of thesubstrate to which the integrated circuit chip is attached. The mold isconstructed so that there is a space between the inner surface of themold and the integrated circuit chip. A metered volume of encapsulantmaterial is dispensed through the opening into the space surrounding theintegrated circuit chip and the space between the integrated circuitchip and the chip carrier or circuit board. The preferred encapsulantmaterial comprises a high strength thermosetting one part epoxycontaining about 50% to 80% by weight of inorganic electricallynon-conductive filler and has a viscosity at 25° C. of about 250Pascal-seconds measured using a Brookfield viscometer, model HBT, with aCP-52 cone head, at 2 rpm; although materials having viscosities in therange of about 10 to 1,000 Pascal-seconds may also be used.

[0010] After the required amount of encapsulant material is dispensed,the material is cured to form a bond between the integrated circuit chipand the chip carrier or circuit board and reinforce the solder ballconnections.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1a is a longitudinal sectional view somewhat diagrammatic ofan integrated circuit chip mounted on a chip carrier;

[0012] FIG 1 b is a longitudinal sectional view somewhat diagrammatic ofan integrated circuit chip mounted on a chip carrier of the ball gridarray type;

[0013] FIG 1 c is a longitudinal sectional view somewhat diagrammatic ofan integrated circuit chip mounted on a chip carrier of the ball gridarray type; and

[0014]FIG. 2 is a longitudinal sectional view somewhat diagrammatic ofan integrated circuit chip mounted on a substrate and covered by a moldready to receive an encapsulant according to one embodiment of thisinvention.

DETAILED DESCRIPTION

[0015] Referring to FIG. 1a, an integrated circuit chip assembly,indicated generally at 12, is comprised of a chip carrier 14, having aremote surface 16 and a mounting surface 18, and an integrated circuitchip 20, having a remote surface 22 and an attachment surface 24. Theintegrated circuit chip 20 is mounted on the chip carrier 14 in astandoff relationship with the attachment surface 24 of the integratedcircuit chip 20 facing the mounting surface 18 of the chip carrier 14,defining a space 28 between the attachment surface 24 of integratedcircuit chip 20 and the mounting surface 18 of the chip carrier 14. In atypical integrated circuit chip assembly, the height of the space 28 isabout 0.002 to 0.006 inches. The attachment surface 24 of the integratedcircuit chip 20 has arranged thereon, a plurality of electrical contacts30. Each electrical contact 30 has a solder ball 32 attached thereto.The mounting surface 18 of the chip carrier 14 has arranged thereon, aplurality of electrical contacts 34, each of said electrical contacts 34arranged to correspond to a solder ball 32 on the attachment surface 24of the integrated circuit chip 20.

[0016] The chip carrier 14 in one embodiment is comprised of a ceramicmaterial, typically alumina having a coefficient of thermal expansion ofabout 6 ppm/° C. The chip carrier can also be comprised of organicmaterials such as PTFE, polyimides, polytetrafluoroethylene, epoxies,triazines, bismaleimides, bismaleimides/triazines, and blends of thesematerials. These materials may be reinforced with either woven ornon-woven inorganic or organic media such as glass or organic fibers.Such organic materials typically have coefficients of thermal expansionranging from about 6 to 50 ppm/° C. The chip carrier has arranged aboutits perimeter, a plurality of electrical contacts 36. Each electricalcontact 36 has attached thereto a wire lead 38 for interconnectionbetween the chip carrier 14 and a circuit board, to which the integratedcircuit chip assembly is to be attached. The chip carrier 14 may also beof the ball grid array type as shown in FIGS. 1b and 1 c, wherein ratherthan having edge leads, solder balls 37 having a diameter of about 0.020to 0.030 inches are attached to the attachment surface 18 or the remotesurface 16 of the chip carrier 14. The integrated circuit chip 20 istypically comprised of monocrystalline silicon having a coefficient ofthermal expansion of about 2 to 4 ppm/° C. Each solder ball 32 istypically comprised of an electrically conductive metallic soldermaterial. The integrated circuit chip 20 is attached to the chip carrier14 by solder reflow. During operation, the chip carrier 14 and theintegrated circuit chip 20 are subjected to repeated cycles of heatingand cooling. Because the chip carrier 14 and the integrated circuit chip20 have different coefficients of thermal expansion, they expand andcontract at different rates. This results in thermal stress on theconnections between the solder balls 32 and the electrical contacts 30and 34, sometimes causing the interconnection between the chip carrier14 and the integrated circuit chip 20 to weaken or even fracture.

[0017] Referring to FIG. 2, in which the several elements are similar tolike elements of FIG. 1, a mold 58 having an inside surface 60 and anoutside surface 62 is placed over the integrated circuit chip 20 so thatthere is a space 70 between the inside surface 60 of the mold 58 and theremote surface 22 of the integrated circuit chip 20, and a void 64surrounding the integrated circuit chip 20. In a preferred embodiment,the mold 58 is comprised of metal or plastic. External pressure isapplied to the outside surface 62 of the mold 58 to seal the mold 58 tothe mounting surface 18 of the chip carrier 14. The mold 58 has at leastone opening 26 extending from the inside surface 60 to the outsidesurface 62 and at least one vent 66. An amount of the encapsulant 40necessary to substantially fill the space 70, the void 64 and the space28 is dispensed through the opening 26. The encapsulant 40 is forcedinto the space 70 and into the void 64 and under the integrated circuitchip 20 into the space 28. In a preferred embodiment, the encapsulant 40comprises Hysol FP-4323, a high strength thermosetting one part epoxycontaining about 50-70% by weight of a ceramic filler and has aviscosity at 250° C. of about 250 Pascal-seconds measured using aBrookfield viscometer, model HBT, with a CP-52 cone head, at 2 rpm,although encapsulants having viscosities in the range of about 10 to1,000 Pascal-seconds can be used. The encapsulant 40 is dispensedthrough the opening 26 using a dispensing apparatus indicated generallyat 42. In the preferred embodiment, using an encapsulant 40 having aviscosity of about 250 Pascal-seconds at 250° C., the dispensingapparatus 42 comprises an injection apparatus with a 0.020 inch diameterneedle. A pressure of approximately 80 psi is required to inject theencapsulant 40 into the void 64 and the space 28. The encapsulant 40 isheated for about 2 hours at 160° C. to cure the encapsulant 40 and forma bond between the integrated circuit chip 20 and the chip carrier 14and reinforce the solder ball connections. The mold 58 can be removedprior to or after curing. This method may also be used to attach anintegrated circuit chip directly to a circuit board.

[0018] Accordingly, the preferred embodiment of the present inventionhas been described. With the foregoing description in mind, however, itis understood that this description is made only by way of example, thatthe invention is not limited to the particular embodiments describedherein, and that various rearrangements, modifications and substitutionsmay be implemented without departing from the true spirit of theinvention as hereinafter claimed. what is claimed is:

1. A method for encapsulating and reinforcing the electricalinterconnections of an integrated circuit chip assembly, comprising thesteps of: providing a substrate having a remote surface and a mountingsurface and an integrated circuit chip having an attachment surface anda remote surface; said attachment surface of said integrated circuitchip being attached to said mounting surface of said substrate in astandoff relationship thereby defining a space therebetween, said spacehaving a height of about 0.002 to 0.006 inches; placing a mold over saidintegrated circuit chip, said mold having at least one opening extendingfrom an inner surface of said mold to an outer surface of said mold andat least one vent, said inner surface of said mold facing said remotesurface of said integrated circuit chip in a standoff relationship,thereby defining a void; applying pressure to said outer surface of saidmold to seal said mold to said mounting surface of said substrate;providing a volume of an encapsulant necessary to substantially fillsaid space and said void, said encapsulant comprising a thermosettingpolymer having a viscosity in the range of about 10 to 1000Pascal-seconds at dispense temperature; dispensing said volume of saidencapsulant through at least one of said openings into said space; andcuring said encapsulant to form a bond between said substrate and saidintegrated circuit chip.
 2. The method of claim 1 wherein saidattachment surface of said integrated circuit chip is attached to saidmounting surface of said substrate using a plurality of solder ballsconnections.
 3. The method of claim 2 wherein said substrate comprises achip carrier.
 4. The method of claim 2 wherein said substrate comprisesa circuit board.
 5. An integrated circuit chip assembly producedaccording to the method of any of claims 1, 2, 3 and
 4. 6. An integratedcircuit chip assembly, comprising: an integrated circuit chip having aperimeter, an attachment surface and a remote surface, a plurality ofconductive contacts arranged on said attachment surface, each of saidconductive contacts having a standoff connection attached thereto; asubstrate having a mounting surface and a remote surface; a plurality ofelectrical contacts arranged on said mounting surface, each of saidelectrical contacts attached to one of said standoff connections; anencapsulant encasing said integrated circuit chip and covering a portionof said mounting surface beyond said perimeter, said encapsulant alsodisposed between said mounting surface and said attachment surface,encapsulating said standoff connections, said electrical contacts, andsaid conductive contacts.
 7. The integrated circuit chip assembly ofclaim 6 wherein said encapsulant comprises a high strength thermosettingpolymer having a viscosity in the range of 10 to 1,000 Pascal-seconds atdispense temperature.
 8. The integrated circuit chip assembly of claim 7wherein said standoff connections comprise solder balls.
 9. Theintegrated circuit chip assembly of claim 8 wherein said substratecomprises a chip carrier.
 10. The integrated circuit chip assembly ofclaim 8 wherein said substrate comprises a circuit board.